The production of needles involves many processes and different types of machinery to prepare quality needles from raw stock. These varying processes and machinery become more critical in the preparation of surgical needles where the environment of intended use is in humans or animals. Some of the processes involved in the production of surgical grade needles include inter alia: straightening spooled wire stock, cutting needle blanks from raw stock, tapering or grinding points on one end of the blank, providing a bore for receiving suture thread at the other end of the blank, flat pressing a portion of the needle barrel to facilitate easier grasping by surgical instrumentation, and curving the needle where curved needles are desired. Generally, extreme care must be taken to ensure that only the intended working of the needle is performed and that the other parts of the needle remain undisturbed.
The strength of the material used to fabricate the needle will, of course, affect the ultimate strength of the needle. The ultimate strength of a curved surgical needle is normally expressed in terms of the tensile strength and bend moment of the needle. By fabricating a needle from a higher strength material, a smaller diameter needle is required to achieve a desired strength. A reduced diameter needle is desirable, since the smaller the diameter of the needle, the less trauma is caused to the tissue through which the needle passes.
Surgical needles have been made from a variety of stainless steels, including 300 series stainless steels (such as, for example, 301, 302 and 304 stainless steel) and 400 series stainless steels (such as, for example, 420 and 455 stainless steel). Other alloys employed in surgical needle manufacture include those described in U.S. Pat. Nos. 5,000,912 and 1,942,150 and in European Patent Application No. 0 294 210.
In general steel making, 455 steel is an example of a precipitation hardening steel. As disclosed in Stainless Steel, R. A. Lula, 25 (1986) (American Society for Metals), the phenomenon of precipitation hardening is most familiar in aluminum alloys. The general principle is to produce a supercooled solid solution from which, on aging, compounds precipitate. Just as it is possible to supercool a liquid solution of salt in water, by cooling it rapidly and carefully, so it is possible to produce a supercooled solid solution. Hence, at room temperature we have a solid solution that is not stable, but mestastable: there are more alloying element atoms in solution than the structure can really put up with. Given the chance, the structure will induce these atoms to form a separate phase. The chance is offered by time and it may be made more attractive by heat.
During the early stages of the precipitation process, the unwelcome atoms move out of the crowded structure and begin to combine with other atoms about them. Eventually they will form precipitates visible under the microscope, but it is during the early stages (before we can readily see what is going on) that the greatest strength increase occurs--during what is called the preprecipitation stage. When visible precipitates have formed, the strength is usually on the decline, and the alloy is said to have overaged.
The precipitates associated with the hardening process are complex. It is, however, easier to state that in the precipitation-hardening stainless steels, additional elements (aluminum, molybdenum, copper) that are active in the precipitation-hardening process are introduced, and heat treatments are devised to produce first the supersaturated solid solution and then the preprecipitation stage.
Prior steel treatments have deficiencies. Hardening the steel in wire form, as in U.S. Pat. No. 5,000,912, makes it more difficult to shape into needles. EP 294210 discloses vacuum heat treating needles of a high cobalt-steel alloy and then cooling them in air.